Optical trapping of absorbing mesoscopic particles in air using photophoretic forces

Sil, Souvik (2022) Optical trapping of absorbing mesoscopic particles in air using photophoretic forces. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Trapping using Optical tweezers (OT) has been conventionally performed in liquids where it is easier to trap particles due to the high viscosity of the fluid. However, trapping absorbing particles in air using OT is quite challenging as the viscosity of air is extremely low - as a result of which the diffusivity of the particle is very high, which requires very steep confining potentials to trap particles in air. Recently, photophoretic forces have provided an alternative route to trap particles in air as these forces are almost five orders of magnitude stronger than the radiation pressure force. In this thesis, we mainly focus on exploring the nature of these photophoretic forces by developing an optical trapping system based on such forces entirely in-house. We then proceed to increasing the robustness of the trap by employing different structured beam profiles, and study the effect of photophoretic forces on the trapped particles in the presence of those beam profiles. We trap absorbing particles in a vertical configuration, where the particles fall under gravity, and the laser light propagates against gravity. When a particle interacts with the laser light, it is heated up to a certain temperature, and the photophoretic forces arise on the particle due to collisions with the air molecules. Thus, while one component of the photophoretic forces results in particle levitation by balancing gravity, the orthogonal component generates a restoring force on the particle to keep it trapped radially by creating a torque on the particle, leading to complex rotational motion of the particle. Hence, the trapping mechanism is rather complex, with a quantitative understanding still under development. Thus, in our first work, we study this orthogonal component of photophoretic forces by trapping a single absorbing particle using a loosely focused Gaussian laser beam and modulating the trap-center spatially using a superposition of multiple sinusoidal frequencies applied to the trapping beam. We determine the resonance frequency and the trap stiffness of a trapped particle by analyzing the amplitude and phase response of the trapped particle at each modulation frequency. Further, we study the variation of the resonance frequency and the trap stiffness as a function of laser power and the intensity experienced by trapped particles of different sizes. In the next work, we use a series of convex lenses of focal length ranging from 25 mm to 125 mm for focusing the trapping beam, and demonstrate experimentally the optical trapping of absorbing multiple particles of different sizes in air employing photophoretic forces and investigate the role of the lenses in the trapping phenomenon. We experimentally determine the dynamic range of optical trapping for each lens system and develop a numerical simulation to explain our experimental observations, which agrees with the experimental results. Further, we move to develop a single optical fiber-based tweezers where we generate a pure Gaussian and a superposition of Gaussian and Hermite-Gaussian beam modes from a single optical fiber that is dual-mode at our operating wavelength. The latter proves to be more efficacious in trapping particles with forces about 1.8 times larger. This result leads to the idea that a multimode fiber with the large radial extent of its intensity profile would be more effective in trapping. This is vindicated experimentally, when we find out that the multimode beam profile gives an eight times stronger trap compared to the Gaussian beam profile. Further, we demonstrate the mode dependency of the radial trapping force of a trapped particle for a multimode fiber as the fiber supports different output modes by changing the input coupling angle. Finally, we provide a basic model based on qualitative analysis and numerical simulations to explain this phenomenon and achieve good agreement between simulation results and those conducted in experiments. Our work in this thesis opens up new possibilities in the fundamental study of photophoretic forces, and also suggests diverse applications in various branches of science including Physics, Chemistry, Atmospheric Science, and Biology.

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